The present invention relates to friction stressed machine parts of cast iron with a ledeburitic bearing crest, such as sealing strips for rotary engines with a ledeburitic bearing crest, and methods for producing them.
In the past, cast iron machine parts which are stressed by sliding friction have been hardened in the region of their bearing surfaces in order to increase their wear resistance. For sealing strips for rotary engines this is done, inter alia, by remelting the bearing crest, either by inductive heating (German Gebrauchsmuster No. 1,898,216), or through the influence of high energy electron beams (German Auslegeschrift No. 2,045,125), and subsequently quenching to produce ledeburitic cast iron with high carbide contents and correspondingly great hardness in the remelted zones. These remelted and quenched zones are comprised substantially entirely of ledeburitic cast iron.
However, with sealing strips for rotary engines, the hardness of the material of the bearing surfaces is not the only factor which is responsible for good wear resistance. The combustion chamber of a rotary engine contains chamber walls which have a trochoidal shape and the sealing strips which are disposed at the corners of the rotary engine rotor contact the trochoidal chamber walls and follow a trochoidal path to seal the combustion chamber. The sealing strips which are disposed at the corner of the rotary engine rotor are known as apex seals, and the portion of a sealing strip which contacts the trochoidal chamber wall is known as the bearing crest or surface. In order for there to be good wear resistance, there must be a good compatibility between the bearing crest of the sealing strip and the bearing surface of the trochoidal chamber wall which is contacted by the sealing strip, and there must be no formation of knocking marks, grooves or burn traces on either the bearing surface of the sealing strip or the bearing surface of the trochoidal chamber wall. However, the known sealing strips, be they made entirely of a ledeburitic solid hard casting, or be they made of gray cast iron having a ledeburitic bearing surface or crest made according to the above-described remelting process, do not always exhibit quite the desired compatibility. The bearing surface of the known sealing strips is very hard and grooves and burn traces form again and again in the trochoidal chamber wall in the path followed by the sealing strips and sometimes this results in malfunction of the entire system of sealing strip and trochoid.
In the ledeburitic bearing crests produced according to the above-described remelting process, the carbon is almost exclusively present as a carbide, lubricating graphite is almost completely absent, and compatibility between the bearing crest of the sealing strip and the bearing surface of the trochoidal chamber wall in the running path followed by the sealing strip is thus reduced during emergency operation (operation during dry run) as well as in normal operation.